When threads are processed or used up, especially in the textile industry, it is frequently important to accurately maintain predetermined thread tensions, for example, to assure the weaving of a textile fabric having a uniform appearance. For this purpose it is necessary to continuously measure or sense the thread tension to quickly recognize any deviations from a rated thread tension so that corrections can be made, for example, by means of a respective closed loop control mechanism for returning the thread tension to the desired rated value. In this context, the thread tensions are rather small, for example, the thread tensions may be smaller than 0.1 N (Newton). Measuring these small tensions accurately has posed a problem in the past.
Furthermore, the threads are usually travelling at substantially high speeds as they are being processed or used, whereby the respective thread tension sensors must have relatively large natural frequencies up to about 3 kHz. Textile machines, such as high speed looms, handle simultaneously a substantial number of threads that may exceed more than 100 threads. Measuring individually the thread tension for each of these threads has posed a space problem for locating the respective large number of thread tension sensors. Accordingly, it is desirable to construct these sensors as compact as possible and also as a mass produceable item at low cost. Yet, the inexpensive sensors shall be able to make precision measurements. Such requirements are not necessarily compatible with each other.
It might be possible to measure the thread tension by means of a spring balance which is connected to an electrical circuit for converting a spring deflection of the balance into a corresponding electrical signal which would then have to be evaluated. For this purpose, it is possible to suspend a thread guide roller from the spring balance which would thus provide a measure for the thread tension. However, such an arrangement has its problems because, the tension to be measured is relatively small, namely less than 0.1 N as mentioned above. The measuring of such small forces would require very soft springs. The respective spring mass system has, as a rule, a very low natural frequency so that such a device would be suitable only for measurements on stationary or very slow moving threads.